This invention relates to transmission of digital signals over communications networks and, more particularly, to measures for improving the accuracy of such transmission.
The transmission of digital data over a bandlimited channel, such as the analog voice channel of the local loop plant can be impaired in different ways. The impairments that most severely limit data rates are: intersymnbol interference, channel noise, analog signal distortion (introduced by analog circuitry in the modem, digital to analog converters, central office line cards, codec filters and the local loop), network impairment and quantization noise. Intersymbol interference (ISI) arises when the frequency spectrum of the transmitted signal is not uniformly accommodated by the channel""s passbandxe2x80x94causing neighboring data symbols in a transmission sequence to spread out and interfere with one another. This problem may be addressed by using an equalizer (a form of filter) to compensate for the channel""s linear (amplitude and phase) distortion. A decision feedback equalizer (FB) may be employed in addition to the linear, or feed forward, equalizer to further reduce ISI and network noise. In conventional practice, the equalizer is trained by using a pre-defined training signal using a small number of constellation points such as a positive and a negative signal of the same absolute value. The problem is that such xe2x80x9ctwo-levelxe2x80x9d training may not be appropriate for the entire range of signal levels and may not be appropriate for all of the different kinds of network impairment. Imperfect adjustment of an equalizer results in a distortion that may be called xe2x80x9cequalizer noisexe2x80x9d.
Channel noise includes thermal or xe2x80x9cwhitexe2x80x9d noise, transients and quantization noise. Quantization noise is inversely dependent on the number of quantization levels. In practical systems where the number of quantization levels is fixed, quantization noise depends on the difference between the quantization levels employed by the digital to analog converter (DAC) at a client (analog) modem at one end of the loop, and the analog to digital converter (ADC) at the network interface to the public switched telephone network (PSTN), at the other end of the loop. A client modem maps specific bit patterns within the user""s data stream to different symbols. The DAC in the client modem converts the symbols to a unique analog signal for transmission over the loop to the network interface which contains an ADC to convert the received analog into digital bit patterns for transmission through the PSTN. The most efficient method to maximize data throughput in the voice telephone network is for the client modem to use the actual slicing levels employed at the network codec and for it to synchronize its transmissions to the network clock. Such a scheme is disclosed in Ayanoglu et al, U.S. Pat. No. 5,394,437 issued to the assignee of the present application. The problem is that ascertaining the slicing levels is not independent of equalizer adjustment.
Network impairment is comprised of a wide range of digital transformations that take place in the telephone network after analog to pulse code modulation (PCM) conversion. For example, the conventional D2 channel-bank pattern employs the 193rd bit of odd frames to provide a repeating pattern 1010 . . . for framing synchronization. The 193rd bit of even frames is utilized to provide a repeating pattern 000111 . . . for identification by 01 and 10 transitions of the sixth and twelfth frames. The eighth bit of each channel may then be preempted for supervisory signaling related to the respective channel. While such xe2x80x9crobbed bitxe2x80x9d signaling may not be noticeable when the channel is carrying ordinary speech, the preemption of a bit position from the data stream injects a certain amount of noise in the channel that can have important ramifications, particularly during equalizer training. While some modems choose a signaling level for equalizer training that is not used for supervisory signaling, some transmission lines may preempt any signaling level for supervisory purposes and therefore inject noise into the channel that will slow down equalizer training.
In the copending application of Dagdeviren-13, Ser. No. 08/829,274, entitled xe2x80x9cSystem and Method for Iteratively Determining Quantization Intervals of a Remote ADC and Modem Employing the Same,xe2x80x9d now U.S. Pat. No. 5,999,564, (the subject matter of which is hereby incorporated by reference), a system is described for determining the actual quantization intervals of the ADC in the network interface and for setting analog signaling levels of the client modem""s DAC to correspond to the actual, rather than ideal, quantization intervals. Briefly, the client modem sends a training sequence of xe2x80x9cprobexe2x80x9d signals to a central device in the network which analyzes the sequence and responds indicating whether the probe signals were higher or lower than the actual thresholds. This system requires two-way transmission of information between the client modem and the central analyzing device.
L. Cai et al, U.S. Pat. No. 5,831,561, issued Nov. 3, 1998 discloses an improvement on the aforementioned Dagdeviren-13 application in which the symbol table employed in the client modem is configurable as a function of the quantization intervals used by the ADC in the network interface so that the maximum possible minimum separation exists between adjacent values.
The above-described prior art systems determine the actual quantization intervals independently of signaling channel equalization (or assume perfect equalization of the signaling channel prior to ascertaining the quantization levels). As a consequence, the compensation for network impairment is only imperfectly achieved because the determination of the quantization thresholds is not independent of the equalizer adjustment and the adjustment of the equalizers is not independent of the slicing thresholds. This gives rise to inaccurate equalizer adjustment which reduces the accuracy of quantization level determination and hence reduces the number of usable PCM levels which, in turn, reduces the data throughput.
In accordance with one aspect of the principles of the present invention, successive procedures are implemented to reduce the effects of the various noise sources and particularly, the noise introduced in a digital communications path by the communications network preempting bit positions from the data stream. The analog modem is provided with an individual slicer table for each bit position which may be subjected to repetitive preemption by the network. Each slicer table is devoted to learning the correct value of a training symbol occurring during a corresponding one of the bit positions. Initially, the channel impairment is ignored and the analog modem""s equalizers are trained in any convenient way, that is, with the slicers frozen at some predetermined level. After the equalizers have been preliminarily adjusted close to convergence, a level-learning procedure is implemented to train the n slicer tables.
Advantageously, the level-learning procedure may employ the 3-part training sequence disclosed in the copending application of the present inventors entitled xe2x80x9cImproved Training of Level Learning Modems,xe2x80x9d filed of even date hereof and assigned Ser. No. 09/337,687. Briefly, a first part of the training sequence is an all-zero or guard interval sub-segment which insulates the current training sequence against intersymbol interference from any preceding sequence. The duration of the guard interval is selected so that any xe2x80x9cspreadxe2x80x9d remaining after a previous sequence will have become sufficiently attenuated. The next part of the sequence is a sub-segment containing one or more level point signals each followed by a set of zero levels sufficient to insulate each level point signal against intersymbol interference from any other signals for that level. This second sub-segment establishes an initial value for the level to facilitate the analog modem""s decision on what the level point is and also to ascertain the channel""s impulse response at that signal level. The third part of the training sequence is a sub-segment which contains additional level points of varying polarity spaced to approximate the maximum repetition rate to be used by the analog modem.
As claimed in the copending application of the present inventors entitled xe2x80x9cApparatus and Method for Adapting a Filter of an Analog Modem,xe2x80x9d Ser. No. 09/338,134, filed of even date hereof, the mean squares error (MSE) between slicer input and output is measures. If the error is within predetermined bounds, the setting of both the feed forward (FF) and feed backward (FB) filters are advantageously frozen, and subsequent (advantageously, the third sub-segments thereof) are used to adaptively update the slicer table to learn the remaining slicer levels. If the mean square error is somewhat larger (but less than some predetermined threshold amount), the taps of the feed backward filter (FB) are frozen and the feed forward filter (FF) is fine tuned for each slicing level ascertained using the channel impulse response obtained for that slicing level, while continuing to update the slicer table until the MSE is below the predetermined threshold. If the mean square error is larger than the predetermined threshold, both the FF and FB filters are fine tuned while continuing to update the slicer table.
In this way the various noise sources are expeditiously dealt with.